Upload iOS peak memory usage metric (#6282)

Summary:
This extracts iOS `Memory Peak Physical, kB` metric and uses it as the benchmark `peak_mem_usage(mb)`. I convert the value to mb because it feels friendlier to human looking at the dashboard, but let me know if we need kb granularity.

Also fix a small regex bug when parsing model name like `tinyllama_xnnpack+custom+qe_fp32` where the backend could include `+`

This also fixes the missing key error in https://github.com/pytorch/executorch/actions/runs/11337625751/job/31531760841#step:7:112, it happened because the script failed to parse `tinyllama_xnnpack+custom+qe_fp32` and got back no benchmark records.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6282

Reviewed By: guangy10

Differential Revision: D64453877

Pulled By: huydhn

fbshipit-source-id: 7d0ad78027a527d9a42243790000787dfcc39e7b